Page:NIOSH Manual of Analytical Methods - Chapter P.pdf/4

 TABLE 1. EXAMPLES OF POTENTIAL UNCERTAINTY COMPONENT SOURCES. Sampling personal sampling pump flow rate: setting the pump and subsequent drift sampling rate of diffusive sampler sampler dimension (aerosol and diffusive sampling) Sample handling sample preparation (e.g., handling silica quasi-suspensions) sample loss during transport or storage Analytical aerosol weighing recovery (e.g, GC-based methods) Poisson counting (e.g., in XRD methods) sensor variation operator effects giving inter-lab differences (if data from several labs are to be used) Evaluation calibration material uncertainty evaluation chamber concentration uncertainty other bias-correction uncertainty Environmental influence parameters temperature (inadequacy of correction, if correction is made as with diffusive samplers) atmospheric pressure humidity aerosol size distribution (if not measured by a given aerosol sampling method) ambient wind velocity sampled concentration magnitude itself (e.g., sorbent loading)

Within the field of industrial hygiene, the quantities uj are often standard deviation component estimates obtained from a single measurement-method evaluation, rather than from replicates. When the estimates are independent, a combined uncertainty uc may be computed (through the propagation of uncertainty approximation) as:

.

(1)

Through a coverage factor k, generally approximated conservatively (e.g., see Technical Note 2 at end of chapter), as equal to 3 for a single method evaluation, an expanded uncertainty U may be computed as:

.

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(2)

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NIOSH Manual of Analytical Methods